Tony Blair has again been forced on the defensive over his decision to continue backing beleaguered Transport Secretary Stephen Byers.

Iain Duncan Smith attacked the prime minister for "rolling out the red carpet" for Mr Byers just minutes after he expressed "regret" for giving an apparently misleading answer on his role in the so-called 'Spingate' affair.

You are too weak to root out dishonesty

Iain Duncan SmithConservative Party leader

But Mr Blair, with Mr Byers sitting to his left, responded by challenging the Conservative leader over what he said were the real issues of getting the rail system running after a botched Tory privatisation.

Both the Conservatives and Lib Dems are angry Mr Byers is still in office, despite admitting he may have given a misleading impression of his role in the departure of his former press chief Martin Sixsmith.

'Fundamentally untrue'

Mr Blair insisted that he expected the "highest standards of propriety" from his ministers.

In feisty exchanges, the Tory leader pressed the prime minister: "Yesterday the transport secretary admitted to this House that he told the British people something that was fundamentally untrue."

Mr Sixsmith insists he did not agree to go

He said Mr Blair, before he came to power, had stated: "I would expect ministers in a government I lead to resign if they lie."

Mr Duncan Smith taunted the prime minister: "If that's so, why is the transport secretary today still in his job on the benches next to you?"

Mr Blair retorted: "I don't accept what you have said," adding that Mr Byers had made an "absolutely full statement".

"Mr Byers and his department should now be allowed to get on with the issues that really matter to people."

Mr Duncan Smith said the prime minister "clearly isn't going to stand by his word", stressing that even rail passengers were petitioning for Mr Byers' removal.

'Scandal and gossip'

The prime minister said the Tories and parts of the media were following the agenda of "scandal and gossip", while the government was concentrating on matters affecting the economy, living standards and transport.

Mr Blair responded that Mr Byers would be judged on the the "real issues" of improvements in the transport system.

Mr Sixsmith has released to the BBC sections of detailed notes he kept throughout the affair, apparently backing his version of events.

Earlier Mr Kennedy argued that efforts to improve the UK's roads and railways would be frustrated unless both Mr Byers and his top civil servant Sir Richard Mottram quit.

Mr Byers told the House of Commons on Tuesday he regretted that comments he made in a Sunday television interview may have suggested he played no role in discussions leading up to Mr Sixsmith's departure.

Mr Sixsmith insists he was forced out of his job over his dealings with Mr Byers' personal spin doctor Jo Moore.

But Mr Byers enjoys the support of much of Labour's backbenches with MPs cheering him during a meeting of the parliamentary party on Wednesday.

Kennedy says Byers position is "untenable"

In the Commons on Tuesday, Mr Byers told MPs there had been a "concerted attempt by a very small number of civil servants in the press office to undermine" his department.

He said he was clear Mr Sixsmith "was not a suitable person to remain in government", but that ultimately, was not his decision as secretary of state.

But Mr Sixsmith's notes, given to BBC2's Newsnight programme, appear to show that Mr Byers was determined to save face by preventing Mr Sixsmith moving to another senior post in Whitehall.

In the notes, Sir Richard is quoted as saying: "The bigger roadblock is Byers. He's invested so much face in this that his credibility is very much on the line."